Synthesis Of Visible-light-active Bismuth Vanadate-based Photocatalysts And Their Photoinduced Charges Properties

Converting solar energy into chemical energy is extremely important to solve theserious energy crisis and environmental issues. Monoclinic bismuth vanadate as alayered oxide has attracted much attention because of its properties. This layeredoxide, which has important applications in ferroelasticity, pigments, and ionicconductivity, also has the ability to photocatalyst degrade pollutants and split waterinto O2with the existence of aqueous solution of AgNO3. These properties makeBiVO₄as a promising functional material with considerable potential application insolving the energy and environmental issues. BiVO₄is very sensitive to the factorssuch as synthesis techniques, the crystal structure and defect contents, so most of thestudies focus on the preparation of BiVO₄with different morphology and crystallinestructure. The photoinduced charge properties including generation, separation, andrecombination were the basic character of functional system. So it is very important tostudy the photoinduced charge transfer properties of photocatalyst in thephotocatalytic process. It is meaningful to further discuss the photocatalyticmechanism and perfect the photocatalytic system. The surface photovoltage （SPV）technique is a well-suitable and more direct method to characterize the behavior ofphotoinduced charge carriers. In this thesis, we studies the photoinduced charge properties of BiVO₄withdifferent crystalline phase and morphology by surface photovoltage （SPV） techniqueand transient photovoltage （TPV） technique at first. And then adjustment the structureof BiVO₄by the deposition of metal or prepared composites. The SPS and phase, fieldinduced photovoltage （FISPS）, and the transient photovoltage （TPV） technologies areused to study the transfer properties of photoinduced charge carriers in differentsamples. The relationship between photocatalytic activity and transfer properties ofphotoinduced charge carriers （including generation, separation, transfer andrecombination） are discussed.Specific work includes the below aspects:1. BiVO₄with different crystal structure was prepared by a coprecipitation andmolten salt method. Monoclinic BiVO₄showed much higher photocatalytic activityboth under the irradiation of UV light and visible light. SPS and TPV were used toinvestigate the detailed transfer properties of photoinduced charge carriers. Theexistence of interfacial electric in mixed crystal BiVO₄changed the transfer propertiesof photoinduced charge carriers.2. BiVO₄photocatalysts with different morphology were synthesized by ahydrothermal process through controlling the addition of surfactant. The sampleswere characterized by XRD, UV-vis DRS, FESEM. The as prepared samples showhigh photocatlytic activity for the degradation of MB under the irradiation of visiblelight. The results indicated that the flower-like BiVO₄showed the enhancement indegradation of MB because of the high separation of photoinduced charge carriers.This study indicates that the suitable structure is one of the important conditions toachieve high photocatalytic activity.3. The Ag-BiVO₄composites were prepared by a hydrothermal process. XRDshowed the existence of Ag and the （121） peak position of Ag-BiVO₄slightly shiftedtoward lower angles than pure BiVO₄. XPS and HRTEM results indicated that the Agnanoparticles existed on the surface of BiVO₄nano-spindly in the state of metallic. SPS and FISPS results indicated that Ag deposition on the surface of BiVO₄acts assink of electrons promoting the separation of photoinduced charge carriers.4. BiVO₄/Bi₂WO₆composite hierarchical clew-like microspheres weresuccessfully synthesized by a simple one-plot hydrothermal route. XRD indicatesthere are only BiVO₄and Bi₂WO₆in the coposites. The SPS and TPV results indicatethat the existence of interfacial electric change the transfer direction of photoinducedcharge carriers and inhibit their recombination. When the molar ratio （W/V） was1:1,the BiVO₄/Bi₂WO₆composite show the best photocatalytic activity for thedegradation of MB under the irradiation of visible light （>420nm）.